Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), and defects in near range vision usually associated with aging (presbyopia). Presbyopia occurs as a person ages when the lens of eye begins to crystalize and lose its elasticity, eventually resulting in the eye losing the ability to focus at near distances, such as the normal reading distance, and in some cases at intermediate distances. Some presbyopic persons have both near vision and far vision defects, requiring bifocal or multifocal lenses, instead of single vision lenses, to properly correct their vision.
A typical single vision contact lens has a focus, which is the point on which parallel rays of light focus when the lens is placed perpendicular to the parallel rays, and an optical axis, which is an imaginary line drawn from the focus to the center of the lens. A posterior surface fits against the cornea and an opposite anterior surface has a vision surface that focuses light to correct the eye's vision. In the case of a typical spherical lens, the vision surface has a single radius of curvature that is the distance from any point on the vision surface to a point on the optical axis referred to as the center of curvature.
A bifocal lens has at least two vision surfaces on the anterior surface of the lens: a distance vision surface, for gazing at far off objects, and a near vision surface, for gazing at close objects (e.g., while reading). Generally, bifocal contact lenses are concentric or segmented in configuration. In a conventional bifocal contact lens of the concentric type, a first, centrally located, circular correction zone constitutes either distant or near vision correction, while a second annular correction zone surrounding the first zone provides the corresponding near or distance vision correction, respectively. In a conventional bifocal contact lens of the segmented or translating type, the lens is divided into two optical zones. Usually the upper zone is for distance vision correction, whereas the lower zone is for near vision correction. With such a translating lens, the distance portion (upper zone) of the lens is in front of the pupil of the eye in straight-ahead gaze, while in downward gaze, the add power or near portion (lower zone) of the lens is over the pupil.
Effective use of a bifocal contact lens requires translation of the eye between vision surfaces when the eye changes from gazing at an object at a distance to gazing at a nearby object. In such a situation, the pupil must move from being subtended by the distance vision surface to being subtended by the near vision surface.
Additionally, with the increasing use of computers, more and more people utilize a computer at work and at home. It is desirable that a translating contact lens could have one or more intermediate vision surfaces in addition to the near and distant vision surfaces. Such a translating contact lens may have to have an ability to control the translation amount of the eye in a manner that the pupil can be subtended by one of near vision surface, intermediate vision surfaces, and distant vision surface.
While there are many designs for hard bifocal translating contact lenses, soft contact lenses have difficulty in translating across the surface of the eye when the visual direction of the eye changes from horizontal gaze (distance vision) to down gaze (near vision). There are several lens designs reported for soft bifocal translating lenses (see, for example, U.S. Pat. Nos. 4,618,277, 5,071,244, 5,371,976, 6,109,749). However, there are some disadvantages associated with those designs in the prior art. Bifocal translating contact lenses in the prior art are not comfortable to be worn. Furthermore, bifocal translating contact lenses in the prior art would be unable to control the translating amount of a contact lens across the surface of the eye.
Therefore, there is a need for a soft translating multi-focal contact lens that is capable of translating across the surface of the eye when the eye changes position from distance vision to near vision and providing wearer's comfort. There is also a need for a soft translating multi-focal contact lens that can control the amount of translation across the surface of the eye when the eye changes position from distance vision to an intermediate vision or to near vision.